1. Field of the Invention
The present invention relates, in general, to a widescreen television signal transmitting/receiving system, and more particularly to a television signal transmitting/receiving system that transmits the time-compressed television signal at the transmitting side and outputs a time-expanded received signal at the receiving side.
2. Description of the Related Art
Recently, in the television broadcasting arts, what is called a wide aspect system is developed. The wide aspect system displays a picture which has a larger aspect ratio than existing NTSC systems. If the wide screen television signal is received without change and displayed by an existing television receiver, a circle will be reproduced as an ellipse in which the length is longer than the width.
Accordingly, as to this wide aspect system, some proposals have been made to assure its compatibility with the existing systems. One of the ways to achieve compatibility is as follows. The wide picture (screen) is divided into a center panel portion and two side panel portions. Further, the signals of the side panel portions are time-division multiplexed within the horizontal blanking period of the signal of the center panel portion, as disclosed in U.S. Pat. No. 4,551,754. By this means, an existing television receiver which has an aspect ratio of 4:3 is capable of reproducing the picture, which has as its aspect ratio 4:3, using only the signal of the center panel portion. The wide picture signal (e.g. the aspect ratio is 16:9) and the multiple signal described above have the same number of scanning lines and field frequency. Therefore, the center panel signal which is divided from the wide picture signal is time-expanded and the side panel signals which are divided from the wide picture signal is time-compressed. Almost all of the existing television receivers which are on sale have an over scanning area in the horizontal direction. Therefore, the reproduced picture of the time-compressed side panel signals appears within the over scanning area. As a result, none of the side panel signals appear on the picture display area of the receiver. But when the wide picture signal is reproduced on the wide screened receiver, the received signal from the transmitter is processed using the reverse of the above procedures and the picture is then reproduced. At the receiving side, the signal of center panel is time-compressed and the signal of side panel portions are time-expanded such as described in U.S. Pat. No. 4,782,383.
In general, considering the viewpoint of frequency domain, the time-expansion process at the receiving side corresponds to the process in which the signal spectrum is compressed to the low side. For example, the signal spectrum from 0 to 4 MHz is time-expanded by the factor of 4. It is then converted to a signal spectrum of from 0 to 1 MHz. This correlation is applied to the noise signal which is superimposed on the television signal during transmitting. For example, noise of a density of n is superimposed on the transmission path band width from 0 to 4 MHz. After the time-expansion by the factor of 4 is performed to the noise spectrum, a noise signal of a density of 4 n is superimposed on the transmission path which has a band width of from 0 to 1 MHz. The visual sensitivity of the high band is dropped because of the noise spectrum. For evaluating the noise spectrum, many kinds of the noise spectrum evaluation curves exist. A curve which is recommended by CCIR (the abbreviation of French word "Comite Consultatif International des Radiocommunications") is one of them.
Thus, when the compressed side panels of a compatible wide screen signal are expanded in a widescreen receiver, there results a noticeable difference between the resolution or high frequency content of the center portion of a displayed widescreen picture and the side panels. This is due to the expanded noise present in the side panel signal portions.